Re: [edk2-devel] [PATCH 1/1] OvmfPkg/VirtNorFlashDxe: fix shadowbuffer reads

["Gerd Hoffmann" <kraxel@redhat.com>]

On Fri, Jan 12, 2024 at 10:47:43AM +0100, Laszlo Ersek wrote:
> On 1/11/24 14:36, Gerd Hoffmann wrote:
> > In some cases (specifically when the flash update region is
> > small but crosses a multiple of P30_MAX_BUFFER_SIZE_IN_BYTES)
> > NorFlashWriteSingleBlock reads only one instead of two
> > P30_MAX_BUFFER_SIZE_IN_BYTES blocks into the shadow buffer.
> > 
> > That leads to random crap being written to the second block,
> > which in turn can corrupt both the variable store and the
> > FTW work space.
> > 
> > This patch fixes the calculation.
> > 
> > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> > ---
> >  OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
> > index 1afd60ce66eb..cdc809d75e3d 100644
> > --- a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
> > +++ b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
> > @@ -566,7 +566,7 @@ NorFlashWriteSingleBlock (
> >                 Instance,
> >                 Lba,
> >                 Offset & ~BOUNDARY_OF_32_WORDS,
> > -               (*NumBytes | BOUNDARY_OF_32_WORDS) + 1,
> > +               (((Offset & BOUNDARY_OF_32_WORDS) + *NumBytes) | BOUNDARY_OF_32_WORDS) + 1,
> >                 Instance->ShadowBuffer
> >                 );
> >      if (EFI_ERROR (Status)) {
> 
> This patch looks like the output of an excellent bug analysis. I'll need
> more time to review this. If you have a ticket with the analysis
> captured (actual numbers, debugging logs, a concrete backtrace / call
> chain triggering the issue, etc), I'd appreciate a reference.

Yea, that was a few days browsing through the flash and Ftw code,
sprinkling in debug logging, learning how all this works.  Have a shell
script resetting a VM in a loop with slowly increasing delay, to trigger
the code paths recovering from interrupted flash writes.

Finding 0xaf (aka PcdDebugClearMemoryValue) in not-yet used places of
the ftw log (which should have been 0xff) was the bit of information
which finally made me find the root cause of the flash corruption.

I have some followup patches I'll send out as patch series in a moment,
one of them adds variables here to make the whole logic easier to
understand.

stay tuned & take care,
  Gerd



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